BRIEF DESCRIPTION OF THE DRAWINGS
Prior Art FIG. 1 is a perspective view of a exemplar conventional barb for a stent in an AAA device; and
Prior Art FIG. 2 shows the reaction of the Prior Art barb of FIG. 1 under deformational loading.
FIG. 3 is a side elevation of an AAA device according to the invention, shown in an expanded state;
FIG. 4 is a perspective view of a barb according to the invention;
FIG. 4A is a broken side elevation of an alternative embodiment of an AAA device according to the invention, shown in an expanded state;
FIG. 5 is a end view of the AAA device according to the invention, shown in an expanded state;
FIG. 6 is a perspective view of the AAA of the invention shown completely within a delivery catheter;
FIGS. 7 and 8 are perspective views of the AAA of the invention shown partially deployed from a delivery catheter;
FIG. 9 is a diagram showing the reaction of the barb of the invention under deformational loading;
FIG. 10 is a laser pattern for cutting barbs on the stent according to the invention; and
FIGS. 11 and 12 are laser pattern for cutting alternate embodiments of barbs according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning now to FIGS. 3 through 5, a vascular endoprosthesis 10 suitable for treatment of an abdominal aortic aneurysm (AAA) includes a main body portion 12 and two smaller leg portions 14, 16 coupled thereto in a generally inverted Y-configuration. Each portion includes a preferably artificial graft 18 and a self-expanding stent 20 coupled relative to the graft. The stent 20 is preferably formed from a superelastic, shape memory material such as Nitinol or other nickel-titanium alloy. At an upper end of the main body portion 12, the struts 21 of the stent 20 define apexes 22 that are provided with barbs 24 for coupling the main body portion to the abdominal aortic wall. Referring to FIG. 4A, the barbs 24a may alternatively or additionally be provided between the struts 21 at one or more intermediate locations along the length of the stent 20 between its ends.
Referring to FIGS. 4 and 5, each barb 24 includes a twisted neck 26 shape set via shape memory effect metallurgy principals. Several axes can be defined relative to each barb 24: an X-axis extending tangential to the circumference of the stent, a Y-axis extending radially outward from the circumference of the stent, and a Z-axis extending through the neck 26 of the barb 24. The shape set biases the neck 26 to rotate about the Z-axis and point an anchor portion 28 of the barb radially outward relative to the longitudinal axis AL of the stent 20 of the main body portion 12 such that the barb extends in alignment with the Y-axis. In a preferred embodiment, each anchor portion 28, as formed, is also oriented substantially perpendicular (e.g., 90°±15°) relative to the Z-axis, even when in the non-deployed configurations of FIGS. 6 and 7.
Referring to FIGS. 6 through 8, the motion to move each barb 22 into a deployed configuration for gripping the abdominal aortic wall is a twisting motion about the Z-axis which twists each barb 22 about its neck 24 into its stress free shape. Such motion moves the anchor portion 28 of each barb 22 from being substantially aligned with the circumference of the stent 20 when the stent is in a compressed non-deployed state within the delivery catheter 30 (FIG. 6) to extending radially outwards from the stent when in an expanded deployed state (from FIGS. 7 to 8, and eventually to the configuration shown back in FIG. 5). This motion causes the anchor to penetrate the tissue substantially perpendicularly, eliminating any acute tissue migration. It is appreciated that the exact angle of penetration can be designed to optimize migration resistance and downward movement of the endoprosthesis 10 under load of blood flow through the graft 18.
An important aspect of the invention is that the opening motion of the barb (deployed by the shaped set twist) is decoupled from the strength of the barb, thereby allowing a much stronger barb. Finite element analysis models indicate that designs according to the invention are substantially stronger than conventional designs. In addition, referring to FIG. 9, under a bending load of suitable force F to cause migration if the barb were not sufficiently strong, the stress and strain are distributed over a wider area and have much lower values than in conventional designs and the barb deflects a smaller angular displacement α3 relative to conventional designs (as seen by comparing α3 relative to α2 in Prior Art FIG. 2). While conventional designs have inherent weaknesses to allow the barb to open, it is possible to stiffen and strengthen the barb in the present design, as discussed below, to minimize α3 (to approach zero) without any loss of opening function. Thus, the present design has a relatively an increased resistance to fatigue.
Beam theory provides that the strength of a beam is primarily affected by its thickness rather than its width. Because the neck 26 of the barb 24 is twisted, the width of the neck becomes its thickness in the radial direction, which provides great advantage for design flexibility. For a given wall thickness of starting stent tubing, by changing the width to which the barbs are cut, the stiffness of the barbs are easily changed.
FIG. 10 shows a laser cut pattern I 10 for cutting a series of apexes and barbs on a shape memory alloy tube (or flat which can be bent into a tube) for creating the stent 20 for use in the endoprosthesis according to the invention. The barb angle β can be modified to optimize the anchoring force and the stress/strain distribution. The radius r of the barb can be modified to accommodate more barbs and to change the stiffness of the barbs. As discussed above, the width w of the barbs can be modified to change the stiffness of the barbs and to lower the strain during forming. By way of one example, and not by limitation, the barb angle β can be 103°, the radius r can be 0.0150 inch and the width w can be 0.0100 inch.
The anchors of the barbs may include reverse barbs which increase pullout force. FIG. 11 shows as laser cut pattern for a reverse barb 212 that is recessed relative to the anchor 28a of the barb 24a, while FIG. 12 shows laser cut pattern for reverse barbs 214, 216 that extend outward from the anchor 28a of barb 24b.
There have been described and illustrated herein embodiments of barb design for a stent. While particular embodiments of the invention have been described with reference to an endoluminal prosthesis, particularly suitable for an AAA device, it is not intended that the invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. Thus, the barb design of the invention may be used on stents intended for other purposes. In addition, shape memory alloys, and preferably Nitinol have been disclosed as preferred materials for use in practicing the invention, it will be understood that other shape memory alloys and other shape set materials including biocompatible plastics may be used as well. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its spirit and scope as claimed.
Patent Application
20080021544
